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Interaction of Actin and Tropomyosin

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A new study published by Cell Press in the Biophysical Journal has brought to light the interaction of slender filaments of actin and the protein tropomyosin. The model prepared by the researchers illustrates the relationship of actin and tropomyosin while providing a clearer picture of their dynamic association.

Tropomyosin and actin together bring about the contraction of muscles in almost all species, actin being one of the most common proteins with its existence in unicellular as well as huge multicellular organisms. A cell’s cytoskeleton is largely composed of actin which brings about changes in its shape, cellular locomotion and helps in muscle contraction. Tropomyosin and actin bind to each other and tropomyosin acts a molecular barrier which covers up actin’s binding sites and thus hinders its interactions with other proteins. As a response to the sabotage caused by tropomyosin, these cells prompt regulatory proteins to relocate tropomyosin, thereby hampering the barrier earlier created. This hampering of tropomyosin dislodges the barrier so as to let the actin associate with other proteins and remodelling.

Dr William Lehman, a scientist in the Department of Physiology and Biophysics at Boston University explained that earlier studies believed that when studied alone, tropomyosin appeared to be a coiled coil which was arranged along the surface of actin filaments and was similar in shape as well. A thorough interpretation of its regulatory mechanisms needs to be done for understanding its atomic structure and its relationship with its biological substrate.

In their study, Dr Lehman and his colleagues worked on previous knowledge of the electrostatic interaction between oppositely charged actin and tropomyosin, which are oppositely charged amino acids. To find out the most favourable interaction between actin and myosin, different combinations of rotations and positions of tropomyosin were studied. To further illustrate the interaction between actin and tropomyosin, electron microscopy was used as an adjunct. The study of rotations and positions as well as electron microscopy showed similar results, thus confirming the competence of the model.

The study conducted by Dr William Lehman concluded that the weak interaction between tropomyosin and actin can be easily disturbed by some regulatory proteins and this may act as a further switch to control the interaction of actin with other proteins. Dr Lehman quoted that the model proposed by him and his colleagues may serve as a reference point for characterizing tropomyosin movements on actin filaments. Further, this map of the structural platform will help in the assessment of mutations that might influence the interaction between actin and tropomyosin. It may even serve as a basis for the development of tropomyosin mimicking peptide drugs for moderating the interactions between actin and myosin or other molecules.

Xiaochuan Edward Li, Larry S. Tobacman, Ji Young Mun, Roger Craig, Stefan Fischer, William Lehman. Atomic Model of F-Actin-Tropomyosin. Biophysical Journal, 2011; 100 (3): 586a DOI: 10.1016/j.bpj.2010.12.3385
published: 21 Nov 2011 (1:57)

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